The present invention relates to a low-loss Darlington-type transistor circuit and more particularly to such a circuit in which the power transistor may operate alternately as an independent transistor or in a Darlington circuit.
FIG. 1 recalls the structure of a two-transistor Darlington circuit. This circuit comprises a power transistor or main transistor T.sub.1 and an auxiliary transistor or pilot transistor T.sub.2. The collectors of these two transistors are interconnected and the base of the main transistor is connected to the emitter of the pilot transistor. This assembly comprises three connection terminals, a collector terminal C corresponding to the collector terminal C.sub.1 of the main transistor, an emitter terminal E corresponding to the emitter terminal E.sub.1 of the main transistor and a control terminal B corresponding to the base B.sub.2 of the pilot transistor. Generally, there will be designated hereinafter without an index number the terminals corresponding to the circuit as a whole, with the index number 1 the terminals corresponding to the main transistor T.sub.1 and with the index number 2 the terminals corresponding to the pilot transistor T.sub.2. A Darlington circuit such as shown in FIG. 1 may be constructed in the form of an integrated structure on a semiconductor substrate or in the form of an association of discrete components. In all cases; the main transistor T.sub.1 is a transistor of high power compared with transistor T.sub.2.
The advantages of a Darlington circuit are well-known and reside essentially in the fact that with this circuit the main transistor T.sub.1 can be made to conduct in the saturated state with a much lower base current than would be necesary in the absence of transistor T.sub.2.
Within the scope of the invention, a disadvantage of the Darlington circuit will be more particularly examined namely, for a current range which transistor T.sub.2 could withstand alone, the forward voltage drop is higher in the case of the Darlington circuit than in the case where this transistor T.sub.1 is mounted alone.
We will be concerned here essentially with the cases of power transistors operating as switches, i.e. in which the base current is sufficiently high for always operating under conditions of real saturation.
Thus, if we consider a transistor such as transistor T.sub.1 mounted independently in which it is desired to obtain a collector current I.sub.C1 of 50 A, so as to have a condition of real saturation the base current will have to be about 2.5 amps when the forward voltage drop between the collector and the emitter will be substantially V.sub.CE sat =0.4 volts. If we now wish transistor T.sub.1 associated in the Darlington circuit with transistor T.sub.2 to operate under the same conditions, i.e., with a collector current of about 50 amps, the base current will only have to be 0.5 A but the forward voltage drop will be greater than 1 volt. The reduction of the base current in this range of values is not an appreciable advantage whereas the increase in the forward voltage drop forms a decided disadvantage. In fact, it corresponds to consumption in the transistor, i.e. heat losses. On the other hand, if it were desired to operate transistor T.sub.1 with a very high collector current of the order of 150 A, this would be impossible because of the very rapid increase of the forward voltage drop in this transistor and, furthermore, even if it were possible, would require the provision of a very high base current, for example of the order of 30 amps. On the other hand, with a Darlington circuit, for a collector current of 150 A, saturation operation could be provided with a reasonable base current of the order of 2 A and the forward voltage drop would be less than 1.8 volts.
FIG. 2 shows forward voltage drop characteristics (V.sub.CE) as a function of the collector current (I.sub.C) for an independent transistor (curve 1) and for the association in a Darlington circuit of the same transistor with a pilot transistor (curve 2). The base currents are not shown but are assumed to be such that the power transistor always operates in a state of true saturation. On the coordinate axes of FIG. 2 have been shown numerical values to help with understanding but these numerical values are not to be interpreted as giving a limiting indication on the scope of the invention. Curves 1 and 2 intersect at a point of intersection 3. For values of the collector current greater than the value corresponding to point 3 (100 A in the example chosen), it is clear that only operation in a Darlington circuit is possible and it is for this reason that Darlington-type circuits are usually used. For collector currents less than those corresponding to point 3, the curves of FIG. 2 show clearly that the consumption or forward voltage drop of the independently-mounted transistor is distinctly less than that of the same transistor in a Darlington circuit. Furthermore, for this range of values, the base currents required for placing an independent transistor in true saturation operation remain reasonable. Thus, it is a feature of the present invention to have clearly brought out this phenomenon illustrated in FIG. 2, i.e. that it is advantageous, insofar as the forward voltage drop is concerned, to have operation as an independent transistor for a first range of values of the collector current then Darlington operation for higher values of this collector current.